APC Drug Interactions and Contraindications Flashcards

Which combination poses the greatest risk of clinically significant QT prolongation?

Answer: Azithromycin and haloperidol

Both azithromycin and haloperidol independently prolong the cardiac QT interval, and their combination greatly increases the risk of torsades de pointes.

Lithium toxicity risk is significantly increased when a patient is also prescribed which drug?

Answer: Hydrochlorothiazide

Thiazide diuretics reduce renal sodium excretion, causing compensatory lithium reabsorption in the proximal tubule and elevated lithium plasma concentrations.

Which drug interaction requires the pharmacist to counsel the patient to avoid grapefruit juice?

Answer: Atorvastatin therapy

Grapefruit juice inhibits intestinal CYP3A4, increasing atorvastatin bioavailability and raising the risk of myopathy and rhabdomyolysis.

A patient on warfarin starts a course of broad-spectrum antibiotics. The pharmacist should counsel them to watch for signs of:

Answer: Enhanced anticoagulation and increased bleeding risk

Broad-spectrum antibiotics reduce gut flora that produce vitamin K, leading to decreased vitamin K levels and enhanced warfarin anticoagulation effect.

Clopidogrel's antiplatelet efficacy is significantly reduced when combined with which drug class?

Answer: Proton pump inhibitors (PPIs)

Many PPIs, especially omeprazole, inhibit CYP2C19 which is required to convert clopidogrel to its active metabolite, reducing its antiplatelet effect.

When counseling a patient about the interaction between tetracyclines and dairy products, the pharmacist explains that dairy:

Answer: Chelates tetracycline reducing its absorption

Calcium in dairy products forms insoluble chelate complexes with tetracyclines in the GI tract, significantly reducing their oral absorption.